Conventional CMOS-based low power and low phase-noise VCO monolithic IC topologies commonly used in RF applications are shown in FIGS. 1, 2 and 3. A conventional LC-tank VCO core 102 shown in FIGS. 1a and 1b is directly biased using a supply voltage (VDD) 104 and a reference voltage (ground) 106 without any biasing component. In the LC-tank VCO core 102, an n-channel MOS transistor pair 116 and a p-channel MOS transistor pair 118 are connected to MOS varactor(s) 112 and inductor(s) 114 for oscillation operation. A control voltage is applied to the MOS varactor(s) 112 to tune the operational frequency of the LC-tank VCO core 102.
In FIGS. 2a and 2b, a conventional LC-tank VCO core 202 is biased using VDD 204 and ground 206 through a p-channel MOS biasing transistor 208 connected between the VDD 204 and the LC-tank VCO core 202. A biasing voltage is applied to the gate terminal of the p-channel MOS biasing transistor 208 to effect control over a biasing current 209 for biasing the LC-tank VCO core 202. In the LC-tank VCO core 202, an n-channel MOS transistor pair 216 is connected across MOS varactor(s) 212 and a pair of inductors 214a and 214b connected in series, the n-channel MOS transistor pair 216, the MOS varactor(s) 212 and the pair of inductors 214a and 214b operating together to provide oscillation. A control voltage is applied to the MOS varactor(s) 212 to tune the operational frequency of the LC-tank VCO core 202. Also, an n-channel MOS transistor pair 216 is employed in the LC-tank VCO core 202 to provide a negative transconductance element to compensate for the resistance loss inherent in the LC-tank VCO core 202.
In FIGS. 3a and 3b, a conventional LC-tank VCO core 302 is biased using VDD 304 and ground 306 through an n-channel MOS biasing transistor 310 connected between the LC-tank VCO core 302 and the ground 306 to provide a biasing current 309. A biasing voltage is applied to the gate terminal of the n-channel MOS biasing transistor 310 to effect control over the biasing current 309. In the LC-tank VCO core 302, a p-channel MOS transistor pair 318 is connected across MOS varactor(s) 312 and a pair of inductors 314a and 314b connected in series, the p-channel MOS transistor pair 318, the MOS varactor(s) 312 and the pair of inductors 314a and 314b together performing oscillation operation. A control voltage is applied to the MOS varactor(s) 312 to tune the operational frequency of the LC-tank VCO core 302. Also, a p-channel MOS transistor pair 318 is employed in the LC-tank VCO core 302 to provide a negative transconductance element to compensate for the resistance loss inherent in the LC-tank VCO core 302.
Due to variations in biasing techniques for conventional VCO circuit topologies, the common limitations in these topologies include poor merit of frequency pushing, which depends on the sensitivity of LC-tank VCO cores to power supplies, and limited utilization of linear range of MOS varactor(s) enabling only limited frequency tuning range of the VCO circuits. There is therefore an apparent need for VCO circuits that provide improved frequency pushing and wider frequency tuning range.